This invention relates to an improved method and apparatus for extruding articles and to improved products made thereby. The invention relates more particularly to the fabrication of an extruded article having a profile as seen in cross section which varies along the length of the article.
The need exists for elongated articles having a cross section or profile which varies along the length of the article. More particularly, in the field of horticulture, for example, it is often desirable to provide a means for staking saplings and young trees with an elongated stake which exhibits different requirements with respect to the profile of the stake at different elevations along its length. A sapling stake is intended to be inserted into the soil and to securely form an anchor for maintaining the sapling at a predetermined attitude. A lower segment of the stake profile should be tapered to facilitate insertion into the soil, but nearer the surface of the soil it should exhibit substantial lateral projected area and cross-sectional dimensions in order to resist lateral displacement in the soil to establish a firm anchor in the soil and corresponding relatively greater strength, i.e. greater section modulus, to resist the maximum bending moment which occurs near the soil surface. On the other hand, a resilience or predetermined flexibility in that segment of the stake above the soil is desired to enable the sapling to flex somewhat during its growth under varying atmospheric conditions. A stake having these differing requirements can be provided by varying the profile of the sapling stake along its length and by fabricating it of a material which exhibits a memory characteristic.
When a sapling is supported by unduly rigid stakes, its trunk is not allowed to flex in a natural manner under wind loads. The result is that the trunk of the tree does not develop proper strength of supporting itself under heavier wind loads later on in its life. By suitably tapering the stake a predetermined flexural resilience is provided which tends to match the natural flexibility of the tapering trunk of the sapling. Thereby, the sapling is allowed to flex naturally in all directions under ambient wind conditions. Its trunk develops appropriate strength, and a healthier young tree results when the stake support is removed. In addition to sapling stakes, other articles such as fence posts and the like have differing profile requirements at different elevations along their lengths. Further desirable characteristics for these articles which are generally maintained outdoors are a resistance to corrosion, fungus, and to decay.
Prior articles intended at least partially to satisfy these requirements have been fabricated of metal or of wood. Metal fence post or stake articles have been formed by the usual well known metal fabricating techniques, and have been relatively expensive, have not exhibited the desired resilience, and are subject to corrosion over a period of time in an outdoor environment. Alternatively, articles of this type are fabricated by woodworking techniques and are conventionally of uniform cross-sectional area, thus being unmatched to the tapering trunk of a sapling. They do not provide the desired flexural resilience and generally do not exhibit the desired overall strength and weathering characteristics. Moreover, wood stakes of uniform cross-sectional area often do not provide sufficient lateral projected area at the soil surface and within the soil. Accordingly, if the soil becomes softened by rain, the stake may readily become displaced laterally and tilt over. Furthermore, unless specially treated, they are readily susceptible to decay and to rot.
These articles may be formed of thermoplastic polymer materials which exhibit the desirable memory, resistance to corrosion and which are not susceptible to decay, fungus and to rot. However, thermoplastic polymer articles are generally fabricated by extrusion or by injection molding. In prior extrusion techniques, a plastic stock material in plasticized or in a liquid form is forced through a die having an orifice of fixed dimensions and having a desired profile or cross-sectional configuration. The article thus extruded has a substantially uniform profile along the entire length of the extruded section.
With respect to injection molding, which involves high pressures, for example, such as 20,000 pounds per square inch, the fabrication of elongated stake or fence post articles requires relatively large, strong expensive dies for injection molding presses. In addition, it is often not possible to produce some of the articles by injection molding because of the need for complex, multiple parting lines in the injection molding die to enable stripping of the product from the molds. Thus, articles having undercuts or multiple fins, for example, could not be stripped from the die without damaging the articles.
Accordingly, it is an object of this invention to provide a method and apparatus for extruding an elongated article having a variable profile along its length.
Another object of this invention is to provide an elongated article formed of a polymer plastic and having a variable profile along the length of the article.
Still another object of the invention is to provide an elongated article of the type described having mechanical characteristics of relative rigidity and relative flexibility at different locations along the length of the article.